Colloidal Synthesis of Nickel Arsenide Nanocrystals for Electrochemical Water Splitting

We report a detailed study on the first colloidal synthesis of NiAs nanocrystals. By optimizing the synthesis parameters, we were able to obtain trioctylphosphine-capped NiAs nanoplatelets with an average diameter of ∼10 nm and a thickness of ca. 4 nm. We then studied the performance of such NiAs nanocrystals as electrocatalysts for electrochemical water splitting reactions, namely, acidic hydrogen evolution reaction (HER) and alkaline oxygen evolution reaction (OER). These nanocrystals were found to be the most HER active ones among the transition metal arsenides reported to date despite exhibiting less than 40 h of stability under benchmark operative conditions (i.e., −10 mA cmgeo–2). When tested as alkaline OER electrocatalysts, our NiAs nanocrystals behaved as a pre-catalyst and transformed superficially into an active Ni-oxy/hydroxide. As a result, NiAs nanocrystals featured an OER activity higher than that of benchmark Ni0 nanocrystals. Noticeably, the OER performance, in terms of , was retained for up to 60 h of continuous operation. The present study highlights how transition metal arsenides, whose structural features could be successfully controlled through a proper tuning of the synthetic parameters, might represent an emerging class of materials for electrocatalytic applications.

: List of the NiAs NCs synthetic attempts, gathering all the Ni precursors, ligands/solvents and operative conditions explored in the study. The optimized reaction conditions for OLAM-and TOP-NiAs NCs are written in bold. Acronyms are defined in the following paragraph. All syntheses utilized tris(dimethylamino)arsine as As precursor. Throughout this study, several ligands such as oleic acid (OA), oleylamine (OLAM), octadecylamine (OCTAM), trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) have been tested in order to optimize the NiAs synthesis route. In the following, the outcomes of all synthetic approaches are commented in the same order in which experiments are presented in Table S1.

Ni precursors
At first, NiCl 2 and tris(dimethylamino)arsine have been chosen as Ni and As precursors, respectively.
OA in ODE: The interaction between the carboxylic group of OA and the aminoarsine leads the formation of As 2 O 3 ( Figure S2a). A possible reason for the formation of As 2 O 3 might be water formation in the reaction flask (through the condensation reaction between OA and aminoarsine) that, combined with the high temperature, leads to the decomposition of the As precursor. To avoid the formation of water as byproduct of the reaction, OLAM might be used as ligand.
OLAM in ODE: Performing the reaction at 220°C for 1 minute yields a NiAs bi-phasic sample composed by nickeline (NiAs) and maucherite (Ni 11 As 8 ) ( Figure S2b). The maucherite phase completely converts into nickeline at longer reaction times (i.e. 10 minutes). By raising the temperature to 250°C we obtain NiAs pure phase samples regardless the reaction time ( Figure S2c, Figure S3e,f). Also, OLAM is a ligand that may be easily handled outside the glovebox, therefore up-scaling of the reaction by the heat-up method is possible under these reaction conditions. This leads to the possibility to synthesize large quantities of NiAs NCs per batch. However, large (34.0 ± 0.5 nm, inset in Figure S3e) and irregular NCs are obtained through this synthetic approach. Note that the synthetic procedure followed when using OLAM as capping agent is identical to the one reported in the Experimental Section but replacing TOP with the same volume of OLAM (70% technical grade, purchased from Sigma-Aldrich).

S6
OLAM: The possibility to use OLAM as both solvent and ligand has also been explored, performing the synthesis at different reaction times ( Figure S2d) or temperatures ( Figure S2e). Regardless the reaction time and/or temperature set, all samples exhibited multiple NiAs phases.
OCTAM and TOPO: When OCTAM ( Figure S2f) or TOPO ( Figure S2g) are used as solvent/ligand, the patterns obtained are not clear and therefore suggest poor phase purity.
TOP and TOP in ODE: The use of TOP as both ligand and solvent (250°C, 1 minute) result in the formation of NiAs but with a non-negligible contamination by As 2 O 3 ( Figure S2h). On the other hand, the use of TOP in ODE (250°C, 1 minute) allows to obtain pure phase ( Figure S2i), ca. 10 nm NiAs nanodisks.
When switching to a different Ni precursor (i.e. Ni(acac) 2 ), the synthesis is unsuccessful irrespective of the solvents, ligands, reaction temperatures and/or times explored ( Figures S2j,k,l).
Sample TEM images of the products of unsuccessful synthetic protocols are reported in Figure S3a

Electrochemical characterization
The following table reports the testing routines applied for HER and OER, providing all the significant analytical parameters. Potential ranges are reported versus the reference electrode (3 M KCl Ag/AgCl).
Regarding the determination of ECSA by double-layer capacitance method and the use of potentiostatic techniques to assess Tafel slopes, additional information may be found in the dedicated paragraphs.
However, since the accurate determination of both ECSA and Tafel slope require ad-hoc optimization of the electrochemical, the reader is recommended to refer to the articles indicated in the following table. 5 CA -Tafel slope determination Refer to ( 2 ) and Figure S14 Refer to ( 2 ) and Figure S33 6 CP -± 10 mA cm -2 Geo I: -2.5 mA (-10 mA cm -2 Geo ) I: 2.5 mA (10 mA cm -2 Geo )

LSV
As reported in point 4 As reported in point 4

PEIS
As reported in point 2 As reported in point 2

ECSA determination by C dl method
To compare the intrinsic electrochemical activity of different systems, the electrochemically active surface area of the electrodes may be estimated through the measure of its specific capacitance. Briefly, CVs of the studied electrode are collected in a non-faradaic region by applying different potential scan rates. The resulting capacitive current is taken as the mean (absolute value) of cathodic and anodic currents registered in the middle of the potential window. Such averaged capacitive current is then plotted against the applied potential scanrate and specific capacitance regressed. Conversion of the specific capacitance of the studied electrode into ECSA is generally performed using the specific capacitance for a flat standard with a 1 cm 2 .
In the present study, in accordance with Gauthier et al. 3 , a thin film of CoP 3 , exhibiting a capacitance of 60 F cm -2 per cm 2 ECSA has been taken as standard. As previously discussed, ECSA determination by the C dl method endows several steps, each one of them to be optimized to the system under study to achieve reliable data. The authors recommend to the reader to refer to dedicated papers. 1

Tafel slope regression from potentio/galvanostatic methods
As discussed in the main text, the regression of Tafel slopes from data collected by means of potentiodynamic techniques is hindered by the unavoidable contribution of capacitive currents. With the aim to minimize such currents, a rapid and effective method for assessing Tafel slopes consist in collecting i vs E data by means of potentio/galvanostatic methods 2 , i.e. chronoamperometric or chronopotentiometric techinques.
Briefly, when using a chronoamperometric technique, several fixed potential steps are set, resulting in a decreasing staircase-like trend of the registered current. The initial step is at the highest potential applied as to promote the formation of the electric double layer at the electrode-electrolyte interface. The acquistion time for each step is set as to achieve a stationary current, therefore eliminating any capacitive contribution.
The same procedure may be applied when using a chronopotentiometric technique, in this case starting from higher current densities, resulting in a staircase-like potential trend. Data for Tafel slope regression are collected by averaging the last few points registered at the end of each step (where capacitive currents are negligible) and then plot as overpotentials versus log i, obtaining the typical Tafel plots. Since each electrochemical system possesses unique capacitive and/or inductive behavior, in addition to peculiar catalytic activity, ad-hoc optimization of the operative parameters (especially the timescale of potentio/galvanostatic steps) has be to carried out. 2 Insets in Figures S14b and S32b report the traces of chronoamperometric/chronopotentiometric scans from which the data for Tafel slope regression has been extracted. Figure S10: Sample conditioning cyclovoltammetric curves (scan rate = 100 mV s -1 ) of NiAs/Toray paper electrodes Figure S11: Replicated LSV curves (scan rate = 2 mV s -1 ) registered on three independent NiAs/Toray paper electrodes in comparison with the bare support. Figure S12: Sample Nyquist plot obtained by PEIS on the NiAs/Toray paper electrode. In the inset, the model circuit used for modeling experimental data and evaluate the system uncompensated resistance R u S19 Figure S13: (a) Cyclovoltammetric curves collected on NiAs/Toray paper electrodes in non-faradaic region at different scan rates; (b) Logarithmic plot of the average I C , determined from CVs reported in (a) at 470 mV vs RHE (IR-corrected) versus the applied voltage scan rate; (c) linear and (d) allometric regression of the averaged I C versus the applied voltage scan rate, yielding the specific capacitance of NiAs/Toray paper electrodes. Table S6: Model equations, regressed parameters and adjusted R 2 coefficient obtained from linear and allometric regression of data reported in Figure S13c and d. Normalization of the delivered current by the electrochemically active surface area (ECSA) allows a fair comparison of the electrocatalytic activity of different materials; however, accurately determining ECSA might be challenging, as proven by the numerous articles and reviews on the topic. 4,5 Among all methods proposed for ECSA determination, the double-layer capacitance is the most used one, mainly because of its universality and simplicity. 1 On the other hand, the accuracy and reproducibility of this method is very poor when compared to other methods (e.g. underpotential deposition of metals and probe molecules stripping methods). 4 The rather limited literature on TMAs generally reports ECSA-corrected data by applying the double-layer capacitance method, 3 so we followed the same approach for our samples. 245 mV dec -1 ). When deriving Tafel slopes from first principles 7 , it appears evident that values > 120 mV dec -1 do not possess any physical meaning, at least when considering the most straightforward reaction mechanism depicted in Scheme 1. Nonetheless, several authors reported HER Tafel slopes > 120 mV dec -S22 1 8,9 , claiming that those values stem from charge transfer coefficients () being lower than the postulated ones ( = 0.5 for Volmer and Heyrovsky steps and  = 0 for Tafel step). It should be also stressed that atypical  values may arise from the complex nature of reaction mechanisms, as commonly reported for electrocatalytic oxidation of organic molecules. 10 Taking into account all these considerations and acknowledging that the theoretical underpinnings of Tafel analysis do not hold universally, 2,6 we may only state that the most reliable 245 mV dec -1 slope determined for NiAs accounts for a hindered HER process on this catalyst and (most likely) on TMAs in general. It must also be stressed out that the measurement of Tafel slopes might be heavily affected by the intrinsic instability of NiAs NCs under acidic HER conditions, as discussed in the main text.

1.4a Alkaline HER: additional performance study
In the following, the NiAs performance under HER alkaline conditions is reported. The whole study has been carried out according to the standard electrochemical procedures reported previously (see the Experimental section in the main text and Table S5). The only differences from acidic HER testing are related to the electrolyte nature (being in this case 1 M aqueous KOH, treated with Chelex resins) and the potential window under study (when the latter is considered versus the Ag/AgCl reference electrode as it is in Table S5). However, all voltages are reported vs RHE, allowing the reader to retrieve the applied potentials vs Ag/AgCl (see the Experimental section in the main text).
Brief comments after each figure/table will summarize the main outcomes of this investigation.      As previously observed for CVs, a double slope is observed in the LSVs as well. Interestingly, the presence of a first, milder slope is detected also on the bare support (blue curve in Figure S21a), corroborating the hypothesis of a pre-HER support reduction. Anyway, the marked increase of this initial slope upon NiAs deposition (black curve in Figure S21a) opens to the possibility of this phenomenon to be also correlated with HER onto NiAs. Being this a complex matter of investigation and being outside from the main objectives of this paper (which focuses on acidic HER on TMAs as possible replacement of PGM-based electrocatalysts), further dedicated studies will be needed to elucidate this phenomenon.
The 4-hours long CP (Figure 21d, average of two replicates, check Figure S22 inset), run at -10 mA cm -2 (geometric current), stabilizes around of 250-300 mV (Table S8). Regarding the long time η HER -10mA cm -2 geo stability ( Figure S22), the outcome of a 40 hours long CP (always run at -10 mA cm -2 geometric current) is consistent with what observed at short scale: the overpotential indeed oscillates between 200 and 300 mV.
The large oscillations reported in Figure S22 are most likely imputable to day-night temperature variations.
As a matter of fact, the electrochemical cell used for the stability test has not been thermally insulated and overpotential fluctuactions come in repetitive "waves" with a ca. 24 hours period. The η HER -10mA cm -2 geo exhibited by NiAs under alkaline conditions is by all means more promising than that recorded under acidic conditions. However, the here reported is still far from recent state-of-the-art η HER -10mA cm -2 geo electrocatalysts for alkaline HER [11][12][13][14] , although several of them have the main drawback of being PGMbased.
Post-CP LSVs ( Figure S23) indicate an almost full retention of the HER activity of NiAs after short operation times (i.e. 4 hours, blue dotted line in Figure S23), while a loss of activity, represented by the decreased i vs E slope after the reaction onset, is detected after longer CPs (i.e. 40 hours, blue dashed line in Figure S23). It is worth noticing that the pre-HER onset slope is still present after long time continuous operation at cathodic currents, further suggesting that this first cathodic phenomenon might be something more complex than the simple support reduction. Figure S24: Tafel slope extracted from chronopotentiometric test. The electrochemical trace from which overpotential versus log i data have been obtained is reported in the inset Going to NiAs microkinetics, the catalyst Tafel slope has been measured according to the above-discussed procedure. Figure S24, as expected, features two regions with different slopes, due to the simultaneous reactions (allegedly, surface reduction of the carbon-based support and actual HER on NiAs). The high current region is the one containing the information related to HER kinetics on NiAs under alkaline conditions. However, the contribution of the support reduction (or, more in general, of the first cathodic phenomenon) cannot be ruled out in these tests. The Tafel slope measured for the sample in 1 M KOH is again > 120 mV dec -1 . The reasons why this might happen have been thoroughly discussed in the previous section. We should also point out that the presence of a parallel faradaic process forces the present Tafel slope to be regressed from a high current region; considering the cell geometry and the absence of forced S34 convection (i.e. no RDE testing have been carried out), diffusional problems might arise at such high currents, hampering a correct determination of kinetic parameters. Figure S25: (a) XRD patterns, (b) XPS survey (top) and HR-XPS of Ni 2p and As 3d regions and (c) SEM images with related Ni and As elemental maps of NiAs/Toray paper electrodes after long CP scans under alkaline HER conditions.
A complete physical-chemical characterization has been carried out on the electrodes after long CP scans.
XRD patterns ( Figure S25a) show that the phase purity of the sample is retained (NiAs reflections still present, no new peaks detected), although a non-negligible decrease in peaks' intensity can be noticed. This phenomenon might indicate either dissolution of NiAs or amorphization. XPS spectra ( Figure S25b

Additional comments on the CVs shapes and peaks
CVs collected on NiAs/Toray paper electrodes show a peculiar shape, characterized by the presence of a faradaic peak at large anodic potentials (ca. 2 V vs RHE) that disappears upon potential cycling. As multiple cycles are recorded, the OER performance of NiAs NCs increases steadily, reaching a stable behavior between cycles 45 to 50. The disappearance of the peak (most likely related to the As leaching through oxidation and related surface reconstruction) and the increase of performances indicate that NiAs is actually the pre-catalysts, transforming during cycling to the real active phase, 16 Table S1) on NiAs NCs, Ni 0 NCs and the bare support

Additional comments on the Ni(II) to Ni(III) oxidation peaks
As is clear from the electrochemical traces reported in Figure S31, the formation of the Ni(III) active site is influenced, in terms of formation energy barrier (i.e. the actual potential of Ni(III) oxidation) by the nature of the Ni-base catalyst. Ni 0 NCs display an oxidation peak centered at ca. 1.45 V vs RHE, consistent with the literature. 17 On the other hand, Ni(II) to Ni(III) oxidation appears to be hindered on NiAs, presenting a slightly larger peak potential (ca. 1.52 V vs RHE, black line in Figure S31)

Turnover frequency (TOF) calculation
TOF is a useful electrochemical key parameter that displays the electrocatalytic activity of a material as the amount of product formed per unit time per given amount of catalyst 4 . Among the numerous equations used to determine the TOF in electrocatalysis, in this paper the following one has been chosen: in which i is the current density (A cm -2 Geo ), x is the number of active sites (mol Active Ni sites cm -2 Geo ), F is the Faraday constant (96485 C mol e -1 ) and n the number of electrons transferred to generate one single molecule of product (4 mol e -1 per O 2 molecule in the case of OER). With x being determinated by integration of the Ni(II) to Ni(III) oxidation peak of Ni-based electrodes, TOF (s -1 mol Active Ni sites -1 ) is generally reported as a function of the applied potential (i.e. retrieving i data from LSV curves at fixed E), as displayed in Figure   S32.

Extended discussion on OER Tafel slopes determination and significance
In terms of kinetic parameters, literature reports OER Tafel slopes for Ni-based catalysts ranging from 50-60 to 120 mV dec -1 6 . On the other hand, Masa et al. obtained a Tafel slope of ca. 59 mV dec -1 for NiAs 17 , although the authors do not clearly indicate whether the regression has been performed on potentiostatically or (most likely) potentiodynamically-collected data. For the sake of completeness, both approaches have been followed in the present paper. When regressing the NiAs Tafel slope from data collected by linear sweep voltammetry, a value of ca. 46 mV dec -1 is obtained ( Figure S33a); such slope is consistent with those reported by Masa et al. for different Ni-metalloid alloys. 17 A more rigorous determination of the Tafel slope by a potentiostatic method ( Figure S33b, details available in the dedicated section) highlights instead a higher slope and its dependence on the overpotential, as often reported for OER catalysts. [18][19][20] For  < 0.55 V, NiAs exhibits a Tafel slope of ca. 120 mV dec -1 ; according to microkinetic analyses and theoretical models, such slope value is observed for large coverages of surface species formed in the step previous to S43 the rate-determining one. 6 Therefore, the single Tafel slope is in this case not sufficient to unveil the exact step representing the bottleneck in the OER mechanism onto NiAs. In the high overpotential region ( > 0.55 V), a steeper value of the Tafel slope is obtained (ca. 225 mV dec -1 ). Although already documented for Pt in 1 M KOH 20 , the almost doubling of the Tafel slope with increasing potential is difficult to be addressed as it could stem from both surface coverage-related issue or simply from diffusional limitations, with the latter likely occurring when achieving large current densities under a typical three-electrode configuration.   Table S10: Atomic percentage of Ni and As from EDS analyses reported in Figure S37 Atomic % by SEM-EDS